The reaction that links two monosaccharides together is CONDENSATION REACTION.
Disacharides are formed when two monosacharide are joined together by the dehydration synthesis reaction which results in a glycosidic bond between the two monosacharide molecule. During the process, the hydroxly group from one monosacharide is joined to the hydrogen atom of the other monosacharide to form water which a byproduct of the reaction.